Bogotá, Colombia — Colombia's FARC guerrillas mark their 44th anniversary this week by mourning the death of their leader, Manuel "Sureshot" Marulanda, and facing potential power struggles for control of the movement.

After four decades of leading a militia that at its peak had nearly 20,000 fighters, Marulanda died in March of a heart attack, the rebels confirmed Sunday.

The death of Marulanda, who was born Pedro Antonio Marín to a peasant family in about 1930, could not have come at a worse time for the guerrilla force that he forged to challenge what he saw as a corrupt and contemptible ruling class.

Raúl Reyes, the FARC's No. 2, was killed March 1 in bombing raid on his clandestine camp across the border in Ecuador, a sign of the Colombian military's increased fighting and intelligence capacity against the guerrillas. Just days later, a second member of the secretariat, Iván Ríos, was murdered in his sleep by his own security guard in a sign of growing discontent among the rebel ranks. On March 26, Marulanda, the founding father of the FARC, died after a life of fighting in the mountains and jungles of Colombia.

Rebel commander Timoleón Jimenez confirmed Marulanda's death in a video broadcast on Telesur, the television network based in Caracas, Venezuela. He said that the FARC would continue its "struggle for political power, the struggle for a socially just society and the struggle for socialism ... in spite of this adversity."

What comes next for the FARC, and Colombia, is uncertain. As Marulanda's replacement, the FARC chose Alfonso Cano, the leader of its political – rather than military – wing. But Mr. Cano, say analysts, may not have the necessary weight to command the rebel army.

If he doesn't, an internal purge could see the rise of the hard-line military wing led by Jorge Briceño, known was "Mono Jojoy," says Adam Isacson, a Colombia analyst with the Center for International Policy in Washington. "The group's smaller, less hard-line units might wither away, leaving behind a hard-line, drug-fueled military rump," Mr. Isacson wrote in an e-mail.

However, if Cano does manage to firmly take charge, Isacson says, the FARC could become more dangerous than under Marulanda. "If the group's decisionmaking process becomes less hidebound and sluggish, it may pose more of a threat on the battlefield," Isacson adds.

What the power shift means for the dozens of high-profile hostages held by the FARC – including three American defense contractors – is also unclear.

The family of Ingrid Betancourt, a French-Colombia citizen who has been held for more than six years, issued a statement Sunday asking Cano, whom they described as "an illustrious and progressive man," to free Betancourt and three other Colombian civilian hostages.

But coming from the political side of the FARC, Cano, whose real name is Guillermo Saenz Vargas, may need to show his strength by striking a tough stance against the government, wanting to hold onto the hostages as leverage.

Nonetheless, Defense Minister Juan Manuel Santos held out the hope that the FARC's new leader would be more inclined to negotiate with the government. "This is a message to Alfonso Cano," he said. "The government has always kept open the door of peace."